Conventionally, Twisted Nematic Method (Hereinafter referred to as TN method) or Super Twisted Nematic Method (Hereinafter referred to as STN method) are generally known as the methods used for reflective liquid crystal displays. A reflective liquid crystal display of any of these methods comprises a liquid crystal cell made by sandwiching a liquid crystal layer by two substrates having transparent electrodes, two polarizers sandwiching the liquid crystal cell and a reflective plate provided on one of the polarizers. With this structure, there are problems such as low brightness and poor visibility of displayed images, which are caused by the light-absorbing characteristics of the two polarizers.
On the other hand, with a guest-host method (Hereinafter referred to as GH method) wherein dichroic dye (guest) is added to liquid crystal (host) and displaying is carried out using the absorption anisotropy is known as a method capable of displaying images of high brightness and good visibility, since polarizers are not necessary with this method.
For example, PCGH (Phase Change Guest Host) method using host of cholesteric liquid crystal as proposed in D. L. White and G. N. Taylor: J. Appl. Phys. 45,4718 (1974) is proposed.
The above described reflective liquid crystal display comprises at least a liquid crystal cell made by sandwiching a liquid crystal layer by two substrates having electrodes and a reflective plate, and to prevent parallax between displayed image on the liquid crystal layer and its shadow on the reflector, a structure comprising a reflective electrode for which the electrode of one of the two substrates doubles as a reflective plate is preferably used.
A reflective electrode is made of a film such as a metal deposited film, and as surrounding objects are reflected in displayed images and the visibility is considerably reduced if the electrode surface is specular, light-diffusing characteristics is provided to the electrode by making the electrode surface convexo-concave by methods such as etching. However, when making electrode surfaces uneven, there are the following problems: deterioration of productivity and increase in producing cost caused by the increase in man-hour for making electrode surfaces uneven; difference, electrode by electrode, of light-diffusing characteristics and deterioration of yield rate because of technical difficulty for making the concaveness and convexness of each electrode same; and problems such as deterioration of the contrast of displayed images because of non-uniformity, which is inevitably caused, of the thickness of a liquid crystal layer.
To solve such problems, a structure wherein a light-diffusing layer is formed on a substrate, which does not have a reflective electrode, of the two substrates is disclosed in Japanese examined patent publication Sho 61-8430. In this publication, as shown in FIG. 2, an upper substrate is made by forming transparent electrode 21 and alignment layer 41 on thin substrate 13, applying polarizer 70 to the surface of the opposite side of substrate 13, forming light-diffusing layer 30 on polarizer 70 and sticking light-diffusing layer 30 to substrate 11 (The adhesion layer is not shown in the figure), and a lower substrate is made by forming specular reflection electrode 24 and alignment layer 42 on substrate 12 in order, and the device is made by sandwiching liquid crystal layer 50 by the upper and the lower substrates. According to the present invention, as it is not necessary to make the surface of specular reflection electrode 24 uneven, the above described problems and non-uniformity of the liquid crystal layer do not occur.
However, to carry out displaying of high quality using the above described structure, it is necessary that light-diffusing layer 30 satisfies the following conditions:
1. having adequate light-diffusing characteristics which is high enough to prevent the specular characteristics of the specular reflective electrode when pixels carry out white displaying and with which the reflection rate of outside light is low enough (with back scattering of low degree) when pixels carry out black displaying. PA1 2. having concaveness and convexness on its surface which are small enough compared with the thickness of liquid crystal layer 50 so that non-uniformity in the thickness of liquid crystal layer 50 is not caused. PA1 3. having thickness small enough compared with the size of one of pixels, so that image diffusion, which causes reduction of image resolution, does not occur. PA1 a first substrate having a transparent electrode; PA1 a second substrate having a specular, light-reflecting electrode provided so that its electrode surface faces the electrode surface of substrate 1; PA1 a liquid crystal layer having dichroic dye and sandwiched by the first substrate and the second substrate; and a light-diffusing layer provided between the first substrate and the liquid crystal layer, and PA1 the light-diffusing layer comprises liquid crystalline polymer having directors of random orientation which varies spatially continuously.
Therefore, the light-diffusing layer obtained by applying, to a substrate of glass or resin, a white coating material wherein a white pigment such as TiO.sub.2, ZnO, BaSO.sub.4 or nylon is dispersed in a binder such as acrylic resin, as shown in Japanese examined patent publication No. Sho 57-42867, tend to cause back scattering of high degree, and it is difficult for such a light-diffusing layer to satisfy the above described condition 1. Further, as powder dust tends to be caused by using a white coating material, there is another problem wherein the yield rate is reduced.
With a light-diffusing layer of crystalline polymer sheet as disclosed in Japanese examined patent publication No. Sho 57-43905, which is another example of the material of a light-diffusing layer, the thickness of the light-diffusing layer when it satisfies condition 1 is 0.1 to 10 mm, which is too high to satisfy condition 3. Particularly, when carrying out displaying of high resolution of at least 5 lines/mm, the reduction of image resolution is seen.
As described above, there is no conventional materials, for light-diffusing layers, which satisfy every necessary conditions, and more preferable materials has been desired.